There has been much research on the development of strong polymer materials for special purposes, and on the safety thereof. However, discarded polymer is now socially recognized as a severe environmental problem all over the world, leading to efforts to develop environmentally-friendly polymer materials.
Environmentally-friendly polymers can be mainly classified into photodegradable and biodegradable polymers. Biodegradable polymers have a functional group that can be decomposed by microorganisms.
Among these polymers, aliphatic polyester polymer has gained the most attention, since it has excellent porosity and an easily-adjustable decomposition characteristic. In particular, polylactic acid (PLA) has a market share of 150,000 tons in the world and expansively covers the field where common plastic is used, for example food packing materials and containers, cases for electronics, and the like. At present, polylactic acid resin is mainly used for disposable products such as food containers, wraps, films, and the like due to its biodegradable characteristics. Examples of polylactic acid resins include those manufactured by American NatureWorks LLC, Japanese Toyota Motors Inc., and the like.
However, since conventional polylactic acid resin lacks formability, mechanical strength, and heat resistance, a thin film made therefrom can be easily destroyed. Further, since it has low resistance against high temperature, a molded product made therefrom can also be distorted at 60° C. or higher.
In order to overcome these problems, polylactic acid resin has been blended with petroleum-based thermoplastic plastics, for example a polycarbonate resin, a polyamide resin, an acrylonitrile-butadiene-styrene (ABS) resin, a polyacetal resin, and the like.
Japanese Patent Laid-Open Publication No. 1999-279380 discloses a polylactic acid resin and an acrylonitrile styrene resin composition, and Japanese Patent Laid-Open Publication No. 2006-70224 discloses a polylactic acid resin and an acrylonitrile-butadiene-styrene resin composition to improve heat resistance. Japanese Patent Laid-Open Publication No. 2006-143772 and U.S. Pat. No. 5,272,221 disclose a method of enhancing biomass by using a polylactic acid-polyamide resin composition. In addition, Japanese Patent Laid-Open Publication No. 2003-147180 and No. 2003-138119 disclose a method of increasing heat resistance by using a polyoxymethylene-polylactic acid resin composition.
Japanese Patent Laid-Open Publication No. 2005-048067 and No. 2006-199743 disclose a method of improving heat resistance by using a polylactic acid-polycarbonate resin composition. However, since the polylactic acid resin and polycarbonate resin have low compatibility, there is limited improvement in properties, and the blend has the problem of a flow mark or welding line degrading the appearance of products.
Macromolecules 20, 904 (1987) and 26, 6918 (1993) disclose crystalline improvement by fusing L-isomeric polylactic acid and D-isomeric polylactic acid. Furthermore, Japanese Patent Laid-Open Publication No. 2007-023083 and No. 2006-241607 disclose inducement of high crystallinity and improvement of thermal stability and mechanical strength by using a stereo-complex polylactic acid.